All technologies have a good side and a dark side. The dark side is often difficult to predict by narrowly focused experts: adding a lane to the freeway ultimately adds to congestion; air conditioners make cities hotter; hospitalization makes many patients sicker. Such unintended results are dubbed "revenge effects" in this unsettling book. The wide range of examples is presented in a way that encourages you to think in ways that will help to reduce future problems. Tenner calls for disciplined vigilance and inspections along with more comprehensively considered design.